Particulate corrosion resistant coating composition

ABSTRACT

A composition comprising a glass-forming binder component and a particulate corrosion resistant component. The particulate corrosion resistant component comprises corrosion resistant particulates having: a CTE p  of at least about 4 and being solid at a temperature of about 1300° F. (704° C.) or greater; and a maximum median particle size defined by one of the following formulas: (a) for a CTE p  of 8 or less, an MP equal to or less than (4.375×CTE p )−10; and (b) for a CTEp of greater than 8, an M p  equal to or less than (−4.375×CTE p )+60, wherein CTE p  is the average CTE of the corrosion resistant particulates and wherein M p  is the median equivalent spherical diameter (ESD), in microns, of the corrosion resistant particulates. Also disclosed is an article comprising a turbine component comprising a metal substrate and a corrosion resistant coating overlaying the metal substrate, as well as a method for forming at least one layer of the corrosion resistant coating adjacent to the metal substrate. The corrosion resistant coating has a maximum thickness defined by one of the following formulas: (3) for a CTE p  of 8 or less, an T c  equal to or less than (1.5×CTE p )−3.5; and (4) for a CTE p  of greater than 8, an T c  equal to or less than (−1.5×CTE p )+20.5, wherein T c  is the thickness, in mils, of the corrosion resistant coating.

CROSS REFERENCE TO RELATED APPLICATION

This patent application is a divisional application of U.S. patentapplication Ser. No. 11/311,137, filed Dec. 20, 2005, now U.S. Pat. No.7,604,867.

BACKGROUND OF THE INVENTION

This invention broadly relates to a corrosion resistant coatingcomposition comprising a particulate corrosion resistant component, anda glass-forming binder component. This invention also broadly relates toan article comprising a turbine component coated with at least one layerof this composition. This invention further broadly relates to a methodfor coating the article with at least one layer of this composition.

In an aircraft gas turbine engine, air is drawn into the front of theengine, compressed by a shaft-mounted compressor, and mixed with fuel.The mixture is burned, and the hot exhaust gases are passed through aturbine mounted on the same shaft. The flow of combustion gas turns theturbine by impingement against the airfoil section of the turbineblades, which turns the shaft and provides power to the compressor. Thehot exhaust gases flow from the back of the engine, driving it and theaircraft forward. The hotter the combustion and exhaust gases, the moreefficient is the operation of the jet engine. Thus, there is incentiveto raise the combustion gas temperature.

The compressors and turbine of the turbine engine can comprise turbinedisks (sometimes termed “turbine rotors”) or turbine shafts, as well asa number of blades mounted to the turbine disks/shafts and extendingradially outwardly therefrom into the gas flow path, and rotating. Alsoincluded in the turbine engine are rotating, as well as static, sealelements that channel the airflow used for cooling certain componentssuch as turbine blades and vanes. As the maximum operating temperatureof the turbine engine increases, the turbine disks/shafts and sealelements are subjected to higher temperatures. As a result, oxidationand corrosion of the disks/shafts and seal elements have become ofgreater concern.

Metal salts such as alkaline sulfate, sulfites, chlorides, carbonates,oxides, and other corrodant salt deposits resulting from ingested dirt,fly ash, volcanic ash, concrete dust, sand, sea salt, etc. are a majorsource of the corrosion, but other elements in the bleed gas environmentcan also accelerate the corrosion. Alkaline sulfate corrosion in thetemperature range and atmospheric region of interest results in pittingof the turbine disk/shaft and seal element substrate at temperaturestypically starting around 1000° F. (538° C.). This pitting corrosion hasbeen shown to occur on critical turbine disk/shaft and seal elements.The oxidation and corrosion damage can lead to premature removal andreplacement of the disks and seal elements unless the damage is reducedor repaired.

Turbine disks/shafts and seal elements for use at the highest operatingtemperatures are typically made of nickel-base superalloys selected forgood elevated temperature mechanical properties such as fatigueresistance. These superalloys have resistance to oxidation and corrosiondamage, but that resistance may not be sufficient to protect them atsustained operating temperatures now being reached in gas turbineengines. Disks and other rotor components made from newer generationalloys may also contain lower levels of chromium, and may therefore bemore susceptible to corrosion attack.

Corrosion resistant coating compositions have been suggested for usewith various gas turbine components. These include aqueous corrosionresistant coating compositions comprising phosphate/chromate bindersystems and aluminum/alumina particles. See, for example, U.S. Pat. No.4,537,632 (Mosser), issued Aug. 27, 1985 and U.S. Pat. No. 4,606,967(Mosser), issued Aug. 19, 1986 (spheroidal aluminum particles); and U.S.Pat. No. 4,544,408 (Mosser et al), issued Oct. 1, 1985 (dispersiblehydrated alumina particles). Corrosion resistant diffusion coatings canalso be formed from aluminum or chromium, or from the respective oxides(i.e., alumina or chromia). See, for example, commonly assigned U.S.Pat. No. 5,368,888 (Rigney), issued Nov. 29, 1994 (aluminide diffusioncoating); and commonly assigned U.S. Pat. No. 6,283,715 (Nagaraj et al),issued Sep. 4, 2001 (chromium diffusion coating). A number ofcorrosion-resistant coatings have also been specifically considered foruse on turbine disk/shaft and seal elements. See, for example, commonlyassigned U.S. Patent Application 2004/0013802 A1 (Ackerman et al),published Jan. 22, 2004 (metal-organic chemical vapor deposition ofaluminum, silicon, tantalum, titanium or chromium oxide on turbine disksand seal elements to provide a protective coating).

Another corrosion resistant coating that has been used comprises analumina pigment in a chromate-phosphate binder having hexavalentchromium (commercially marketed by Sermatech International as SermaFlow®N3000). While such a hexavalent chromium-containing coating is effectiveat low temperatures, it has a lower coefficient of thermal expansionrelative to the underlying metal substrate (e.g. superalloy) so that atthe higher temperatures experienced by newer gas turbine engines, thishexavalent chromium-containing coating may spall, even when applied atthicknesses of as thin as 0.5 to 2.5 mils (12.7 to 63.5 microns). Infact, at thicknesses of greater than 1.5 mils (38.1 microns), thiscoating may delaminate after one thermal cycle at 1300° F. (704° C.).While this delamination problem is most evident on the newer highperformance gas turbine engines, this problem may also occur with othergas turbine engines because of the temperature extremes dictated byengine operation.

While these prior corrosion resistant coatings may provide corrosionprotection for turbine disk/shaft and seal elements, there remains aneed for improved corrosion resistant coatings that address thedisadvantages of these prior corrosion resistant coatings, including:(1) possible adverse affects on the fatigue life of the turbinedisks/shafts and seal elements, especially when these prior coatingsdiffuse into the underlying metal substrate; (2) potential coefficientof thermal expansion (CTE) mismatches between the coating and theunderlying metal substrate that may make the coating more prone tospalling; and (3) requiring more complicated and expensive processes(e.g., chemical vapor deposition) for applying the corrosion resistantcoating to the metal substrate.

BRIEF DESCRIPTION OF THE INVENTION

An embodiment of this invention broadly relates to a compositioncomprising:

-   -   a glass-forming binder component; and    -   a particulate corrosion resistant component comprising corrosion        resistant particulates having:        -   a CTE_(p) of at least about 4 and being solid at a            temperature of about 1300° F. (704° C.) or greater; and        -   a maximum median particle size defined by one of the            following formulas:            -   (1) for a CTE_(p) of 8 or less, an M_(p) equal to or                less than (4.375×CTE_(p))−10; and            -   (2) for a CTE_(p) of greater than 8, an M_(p) equal to                or less than (−4.375×CTE_(p))+60.    -   wherein CTE_(p) is the average CTE of the corrosion resistant        particulates and wherein M_(p) is the median equivalent        spherical diameter (ESD), in microns, of the corrosion resistant        particulates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a portion of the turbine sectionof a gas turbine engine.

FIG. 2 is a sectional view of a corrosion resistant coating of thisinvention deposited on the metal substrate of a turbine component.

FIG. 3 is a frontal view of a turbine disk showing where the corrosionresistant coating of this invention can be desirably located.

FIG. 4 is a schematic view similar to FIG. 2 of a corrosion resistantcoating of this invention comprising a plurality of layers.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term “particulate” refers to a particle, powder,flake, etc., that inherently exists in a relatively small form (e.g., amaximum particle size of about 62.5 microns or less, and typically amedian ESD of about 25 microns or less) and may be formed by, forexample, grinding, shredding, fragmenting, pulverizing, atomizing, orotherwise subdividing a larger form of the material into a relativelysmall form. Particulates useful in the corrosion resistant particulatecomponents herein may have a maximum particle size in the range of fromabout 18.5 or less, to about 62.5 microns or less, and a maximum medianparticle size (as determined by the median ESD) in the range of fromabout 7.5 microns or less, to about 25 microns or less, more typicallyin the range of from about 3.8 microns or less, to about 12.5 microns orless. The maximum particle size of the particulates useful herein isprimarily dependent on the CTE_(p) thereof and is abbreviated herein as“A_(p)” and is defined herein in terms of microns.

As used herein, the term “equivalent spherical diameter” (ESD) refers toa diameter of a sphere having the same volume as that of an irregularparticle. See commonly assigned U.S. Pat. No. 6,544,351 (Wang et al.),issued Apr. 8, 2003, the relevant disclosures of which are incorporatedby reference. Particles useful herein may have a spherical shape, nearlyspherical shape, irregular shape, etc., or any combination of shapes.

As used herein, the term “median ESD” refers to the ESD for which 50%(by volume) of the population of particles have an ESD below that value.See commonly assigned U.S. Pat. No. 6,544,351 (Wang et al.), issued Apr.8, 2003, the relevant disclosures of which are incorporated byreference. The maximum median ESD of the corrosion resistantparticulates useful herein is primarily dependent on the CTE_(p) thereofand is abbreviated herein as “M_(p)” and is defined herein in terms ofmicrons.

As used herein, the term “unimodal particle size distribution” refers toa particle size distribution comprising one particle size fraction. Whengraphically plotted (i.e., by particle count as a function of particlesize), a unimodal particle size distribution has essentially a singlepeak.

As used herein, the term “bimodal particle size distribution” refers toa particle size distribution that comprises a larger particle sizefraction and a smaller particle size fraction. When graphically plotted(i.e., by particle count as a function of particle size), a bimodalparticle size distribution has essentially two distinct peaks. Bimodalparticle size distributions provide a greater solids packing density forthe corrosion resistant particulate component. For bimodal particle sizedistributions useful herein, the larger particle size fraction maycomprise particulates having a median ESD at least about 5 times(typically in the range of from about 7 to about 10 times) that of themedian ESD of the particulates comprising the smaller particulate sizefraction. For bimodal particle size distributions useful herein, thelarger particle size fraction typically comprises from about 60 to about95% by volume of the corrosion resistant particulate component, whilethe smaller particle size fraction typically comprises from about 5 toabout 40% by volume of the corrosion resistant particulate component.

As used herein, the term “polymodal particle size distribution” refersto a particle size distribution that comprises three or more particlesize fractions. When graphically plotted (i.e., by particle count as afunction of particle size), a polymodal particle size distribution hasthree or more distinct peaks.

As used herein, the term “solid at a temperature of about 1300° F. (704°C.) or greater” refers to a particulate comprising a ceramic, metal,etc., and combinations thereof that are solid (i.e., do not melt or arenot molten) at a temperature of about 1300° F. (704° C.) or greater(e.g., above the melting point of aluminum), and typically are solid ata temperature of about 1400° F. (760° C.) or greater. The particulartemperature at which the particulates should be solid will depend, atleast in part, on the maximum operating temperature of the environmentthat the coated component is exposed to, and is desirably in excess ofthe maximum operating temperature of the environment that the coatedcomponent is exposed to.

As used herein, the term “substantially free” means the indicatedcompound, material, component, etc., is minimally present (e.g., traceamount) or not present at all, e.g., at a level of about 0.5% or less,more typically at a level of about 0.1% or less, unless otherwisespecified.

As used herein, the term “metal” can refer to a single metal (other thansolely aluminum) or a metal alloy, i.e., a blend of at least two metals(for example, may include nickel-aluminum alloys, etc.). Metals mayinclude chromium, zirconium, nickel, cobalt, iron, titanium, yttrium,magnesium, platinum group metals (e.g., platinum, palladium, rhodium,iridium, etc.), hafnium, silicon, tantalum, lanthanum, etc., alloys ofany of these metals, and alloys of any of these metals with aluminum,e.g., overlay metal alloys. Illustrative corrosion resistant alloysuseful herein may include, but are not limited to overlay metal alloys,other corrosion resistant superalloys with chromium or aluminum levelsabove that of R88DT (16.0 wt % Cr, 2.1 wt % Al), such as IN601 (23.0 wt% Cr), IN625 (21.5 wt % Cr), IN718 (19 wt % Cr), GTD222 (22.5 wt % Cr),Hastelloy X (22.0 wt % Cr), Udimet 500 (19.0 wt % Cr, 3.0 wt % Al), RenéN2 (6.6 wt % Al), René 77 (4.3 wt % Al), MarM 509 (23.5 wt % Cr), HS 188(22.0 wt % Cr), etc.

As used herein, the term “ceramic” refers to an oxide, carbide, nitride,etc., of a metal. Ceramics suitable for use herein may include oxidescarbides, nitrides, etc., of any of the metals referred to herein,combinations of such oxides, carbide, nitride, etc., including, but notlimited to zirconia and phase-stabilized zirconias (i.e., various metaloxides, for example, yttrium oxides blended with zirconia), such asyttria-stabilized zirconias, ceria-stabilized zirconias,calcia-stabilized zirconias, scandia-stabilized zirconias,magnesia-stabilized zirconias, ytterbia-stabilized zirconias, etc., aswell as mixtures of such stabilized zirconias. See, for example,Kirk-Othmer's Encyclopedia of Chemical Technology, 3rd Ed., Vol. 24, pp.882-883 (1984) for a description of suitable zirconias. Suitableyttria-stabilized zirconias can comprise from about 1 to about 65%yttria (based on the combined weight of yttria and zirconia), and moretypically from about 3 to about 10% yttria. Other suitable ceramics foruse herein may include alumina, chromia, silica, titania, ceria,magnesia, hafnia (including phase stabilized hafnias such asyttria-stabilized hafnia), yttria aluminum garnet (YAG), lanthanumhexaluminate, and other metal aluminates, chromium carbide (Cr₂C₃),etc., or combinations thereof Suitable ceramics for use herein willhave: (1) a corrosion resistance greater than that of an R88DT alloy;and (2) a CTE that is at least equal to, and more typically greaterthan, the CTE of alumina.

As used herein, the term “overlay metal alloy” refers to metal alloyshaving the formula MCr, MAl, MCrAl, MCrAlX, or MAlX, wherein M isnickel, cobalt, iron, etc., or an alloy thereof, and wherein X ishafnium, zirconium, yttrium, tantalum, platinum, palladium, rhenium,silicon, lanthanum, etc., or a combination thereof. Typically, theoverlay metal alloys used herein are MCrAlY alloys, and more typicallywherein M is nickel, cobalt, or a nickel-cobalt alloy and wherein X isyttrium (i.e., Y). Illustrative MCrAlY alloys useful herein may include,but are not limited to those comprising 22.0 wt % Cr, 10.0 wt % Al, 1.00Y, the balance being nickel (i.e., M is nickel), or 32.0 wt % Ni, 21.0wt % Cr, 8.0 wt % Al, 0.50 Y, the balance being cobalt (i.e., M isnickel-cobalt alloy).

As used herein, the term “corrosion resistant particulate” refers toparticulates comprising metals, ceramics or combinations thereof thatprovide greater resistance than that of the metals comprising thesubstrate against corrosion caused by various corrodants, includingmetal (e.g., alkaline) sulfates, sulfites, chlorides, carbonates,oxides, and other corrodant salt deposits resulting from ingested dirt,fly ash, volcanic ash, concrete dust, sand, sea salt, etc., attemperatures greater than of about 1000° F. (538° C.), and typicallyabout 1300° F. (704° C.) or greater (e.g., above the melting point ofaluminum).

As used herein, the term “corrosion resistant coating” refers tocoatings that, after curing of the deposited corrosion resistant coatingcomposition, comprise at least one layer having an amorphous, glassymatrix and having embedded therein, encapsulated therein, enclosedthereby, or otherwise adhered thereto, particulates from the corrosionresistant particulate component. Embodiments of the corrosion resistantcoatings of this invention can provide resistance against corrosioncaused by various corrodants, including metal (e.g., alkaline) sulfates,sulfites, chlorides, carbonates, oxides, and other corrodant saltdeposits resulting from ingested dirt, fly ash, volcanic ash, concretedust, sand, sea salt, etc., at temperatures of about 1000° F. (538° C.)or greater, typically about 1300° F. (704° C.) or greater (e.g., abovethe melting point of aluminum), and comprise ceramics, metals, etc., andcombinations thereof. The embodiments of the corrosion resistantcoatings of this invention may be homogeneous or substantiallyhomogeneous throughout in the terms of the composition of theparticulate and binder components, or may comprise a discrete layer(s)that comprises a homogenous or substantially homogeneous composition ofthe particulate and binder components. For example, some embodiments ofthe corrosion resistant coatings of this invention may be a single layercomprising particulates throughout that have a particular CTE value, ormay be a plurality of layers of differing composition, e.g., an innerlayer adjacent to the metal substrate that comprises particulates havinga higher CTE value to more closely match the CTE of the metal substrate,an intermediate layer (or layers) that comprises particulates having alower CTE value that is compatible with the CTE of the inner layer, andan outer layer that consists essentially of a composition that issimilar to a glass-forming binder component but without particulates,e.g., a sealant composition that forms a glassy top coat. The maximumthickness of the corrosion resistant coatings useful herein areprimarily dependent on the CTE_(p) of the corrosion resistantparticulates. The thickness of the corrosion resistant coating isabbreviated herein as “T_(c)” and is defined herein in terms of mils.

A used herein, the term “glass-forming binder component” refers to acomponent comprising a typically inorganic compound, composition, etc.,that, when cured, forms an amorphous, glassy matrix to which theparticulates in the particulate component are embedded in, areencapsulated in, are enclosed by, or otherwise adhered to. Bindercomponents suitable for use herein typically comprise a phosphatebinder, with or without other binder materials. These phosphate bindersmay be in the form of phosphoric acid or more typically the respectivephosphate compounds/compositions, including orthophosphates,pyrophosphates, etc. These phosphate compounds/compositions may bemonobasic, dibasic, tribasic or any combination thereof.Phosphate-containing binder components may comprise one or more metalphosphates, including aluminum phosphates, magnesium phosphates,chromium phosphates, zinc phosphates, iron phosphates, lithiumphosphates, calcium phosphates, etc, or any combination thereof.Typically, the phosphate-containing binder component comprises analuminum phosphate, a magnesium phosphate, a chromium phosphate, or acombination thereof. The phosphate-containing binder component mayoptionally comprise other binder materials, including one or morechromates, molybdates, etc. See, for example, U.S. Pat. No. 3,248,249(Collins, Jr.), issued Apr. 26, 1966; U.S. Pat. No. 3,248,251 (Allen),issued Apr. 26, 1966; U.S. Pat. No. 4,889,858 (Mosser), issued Dec. 26,1989; U.S. Pat. No. 4,975,330 (Mosser), issued Dec. 4, 1990, therelevant portions of which are incorporated by reference. Thephosphate-containing binder component can also be substantially free ofother binder materials, e.g., a substantially chromate freephosphate-containing binder component. See, for example, U.S. Pat. No.6,368,394 (Hughes et al), issued Apr. 9, 2002 (substantially chromatefree phosphate binder component), the relevant portion of which isincorporated by reference.

As used herein, the term “liquid carrier component” refers to anycarrier component that is liquid at ambient temperatures and in whichthe corrosion resistant particulate component and glass-forming bindercomponent is typically carried in, dispersed in, dissolved in, etc.Liquid carrier components include aqueous systems (e.g., comprisingwater), organic systems (e.g., comprising alcohols such as ethanol,propanol, isopropanol, etc., other liquid organic materials or solventssuch as ethylene glycol, acetone, etc.) or any combination thereof.These liquid carrier components may comprise other optional materialssuch as surfactants, buffers, etc. Aqueous carrier components mayconsist essentially of water, i.e., is substantially free of otheroptional materials, but more typically comprises other optionalmaterials such as compatible organic solvents, surfactants, etc.Suitable surfactants for use in aqueous carrier components may includenonionic surfactants, anionic surfactants, cationic surfactants,amphoteric surfactants, zwitterionic surfactants, or any combinationthereof. Illustrative examples of surfactants suitable for use hereininclude ethoxylated alkyl phenols or aliphatic alcohols such as thosesold under various trade names or trademarks including Igepal, Levelene,Neutronyx, Surfonic and Triton, nonionic tertiary glycols such asSurfynol 104, cationic secondary and tertiary amines of the polyoxycocamine type exemplified by Armak Ethomeen C/20 and Emery 6601,quaternary amines such as Armak Ethoquad R/13-50, as well as sodiumheptadecyl sulfate, sodium tetradecyl sulfate and sodium 2-ethylhexylsulfate. The inclusion of surfactants may be for the purpose ofimproving the wettability of the particulate component, reducing thesurface tension of the corrosion resistant coating composition,promoting the formation of improved smoothness in the resultantcorrosion resistant coating, etc.

As used herein, the term “corrosion resistant coating composition”refers to any embodiment of the coating composition of this inventioncomprising the corrosion resistant particulate component, theglass-forming binder component, optionally a liquid carrier component,etc., and which is used to form at least one layer of the corrosionresistant coating of this invention. For embodiments of corrosionresistant coating compositions of this invention, the ratio of thecorrosion resistant particulate component to glass-forming bindercomponent is typically in the range from about 0.1 to about 10, moretypically in the range of from about 0.5 to about 5. The optional liquidcarrier component, when included, typically comprises the balance of thecorrosion resistant coating composition of this invention. Theembodiments of the corrosion resistant coating compositions of thisinvention may be formulated as flowable solids (e.g., flowable powders),may be formulated as cast tapes comprising a blend, mixture or othercombination of the particulate and binder components, with or without asupporting structure such as a film, strip, etc., or may be formulatedas liquids. The embodiments of the corrosion resistant coatingcompositions of this invention may comprise other optional componentssuch as colorants or pigments, viscosity modifying or controllingagents, etc. Typically, the embodiments of the corrosion resistantcoating compositions of this invention are formulated as liquidcompositions. The embodiments of the liquid corrosion resistant coatingcompositions of this invention may be of any desired consistency,flowability, viscosity, etc., including thixotropic or non-thixotropiccompositions. The embodiments of the aqueous corrosion resistant coatingcompositions of this invention often have an acidic pH (i.e., belowabout 7). For example, for embodiments of the aqueous corrosionresistant coating compositions comprising a phosphate-containing bindercomponent, the pH is typically in the range of from about 0 to about 3,and more typically in the range of from about 1 to about 3.

As used herein, the term “curing” refers to any treatment condition orcombination of treatment conditions that causes the corrosion resistantcoating composition to thereby form the corrosion resistant coating.Typically, curing occurs by heating the corrosion resistant coatingcomposition at a temperature of at least about 250° F. (121° C.), moretypically at a temperature of at least about 500° F. (260° C.).

As used herein, the term “turbine component” refers to any turbinecomponent that comprises a metal substrate (i.e., the substrate isformed from metals or metal alloys), and includes turbine componentscomprising airfoils (e.g., blades, vanes, etc.), turbine disks (alsoreferred to sometimes as “turbine rotors”), turbine shafts, turbine sealelements that are either rotating or static, including forward,interstage and aft turbine seals, turbine blade retainers, other staticturbine components, etc. The turbine component for which the embodimentsof the corrosion resistant coatings of this invention are particularlyadvantageous are those that experience a service operating temperatureof at least about 1000° F. (538° C.), more typically at least about1200° F. (649° C.), and typically in the range of from about 1200° toabout 1600° F. (from about 649° to about 871° C.). These components areusually exposed to compressor bleed air or gas path environments havingingested corrosive components, typically metal sulfates, sulfites,chlorides, carbonates, etc., that can deposit on the surface of thecomponent. The embodiments of the corrosion resistant coatings of thisinvention are particularly useful when formed on all or selectedportions of the surfaces of the component, such as the surfaces ofturbine disks/shafts and turbine seal elements. For example, the rim andblade slots of the hub of a turbine disk (e.g., perimeter) may have thecorrosion resistant coating of this invention, while the bore region andinner portion of the turbine disk may or may not have this coating. Inaddition, the contact points or mating surfaces between these componentssuch as the disk post pressure faces, as well as the contact pointsbetween the disks, shafts and/or seals, may be void or absent of thecorrosion resistant coating so as to retain desired or specified asproduced dimensions, but do not necessarily need to be void or absent ofthe coating.

As used herein, the term “CTE” refers to the coefficient of thermalexpansion of a material with reference to a temperature of about 1200°F. (649° C.) unless otherwise specified, and is referred to herein inunits of 10⁻⁶/° F. For example, alumina which has a coefficient ofthermal expansion of about 4 to 5×10⁻⁶/° F. at about 1200° F. (649° C.)is referred to herein as having a CTE of about 4 to 5. The average CTEof the corrosion resistant particulates comprising the corrosionresistant particulate component is referred to herein by theabbreviation “CTE_(p).” Materials useful for corrosion resistantparticulates herein have CTE_(p) values of at least about 4, andtypically in the range of from about 4 to about 12.

As used herein, the term “comprising” means various particulates,materials, coatings, compositions, components, layers, steps, etc., canbe conjointly employed in the present invention. Accordingly, the term“comprising” encompasses the more restrictive terms “consistingessentially of” and “consisting of.”

All amounts, parts, ratios and percentages used herein are by weightunless otherwise specified.

Corrosion resistant coating compositions comprising corrosion resistantceramics and/or metal particulates and phosphate-containing bindersystems, with or without additional chromate binders or other bindermaterials, may be used to provide corrosion resistant coatings forturbine seals and other turbine components such as turbine disks andshafts. The ability to easily and inexpensively form such corrosionresistant coatings over metal substrates of turbine components such asturbine seals, turbine disks, turbine shafts and turbine blades makesthem desirable. For example, these compositions can be delivered byrelatively easy and inexpensive techniques, for example, by spraying anaqueous coating composition comprising the corrosion resistantparticulates and phosphate-containing binder system (with or withoutother binder materials) over or on the metal substrate of the component,followed by heating to a curing temperature of, for example, at leastabout 250° F. (121° C.), more typically at least about 500° F. (260° C.)to provide a corrosion resistant coating comprising corrosion resistantparticulates adhered to or within a glassy phosphate-containing bindermatrix.

For turbine seals and other turbine components such as turbine disks andshafts requiring such corrosion resistant coatings, it has beendiscovered that the size of the corrosion resistant particulates used,as well as the coating thickness, should be controlled based on theCTE_(p) of the corrosion resistant particulate to avoid or minimizeproblems, such as spallation, while at the same time providing effectivecorrosion resistance for the underlying metal substrate. Many metalsubstrates comprise metals or metal alloys (e.g., superalloys) that haveCTEs equal or close to about 8 (e.g., in the range of from about 7 toabout 9). As the CTE_(p) of the corrosion resistant particulates variesfrom about 8, e.g., decreases to about 4 (i.e., lower CTE particulates)or increases to, for example, about 12 (i.e., higher CTE particulates),the potential for CTE mismatches with the underlying metal substrateincrease, thus increasing the potential for spallation of the coating.For example, when using alumina particulates having a CTE of about 4 or5, a relatively thin coating thickness of about 2.5 mils (63.5 microns)or less, more typically to about 1.5 mils (38.1 microns) or less, may benecessary to avoid spallation due to the CTE mismatch with the metalsubstrate.

To provide greater control in obtaining coatings having such relativelythin thicknesses (e.g., about 8.5 mils (215.9 microns) or less forcoatings comprising particulates with CTEs at or about 8, to about 2.5mils (63.5 microns) or less for coatings comprising particulates withCTEs at or about 4 or 12), it may also be desirable to deposit thecoating composition as a plurality of very thin layers. Unfortunately,the particulates in corrosion resistant coating compositions may varygreatly in particle size and in particle size distribution. For example,corrosion resistant coating compositions comprising alumina particulatesmay have particles ranging in size from about 0.1 to about 10 microns orlarger. To achieve effective coating protection with compositions havingalumina particulates of about 10 microns or larger (i.e., at least 1particulate between the coating surface exposed to the corrodants andthe metal substrate surface), the coating may need to be applied at athickness of about 0.5 mils (12.5 microns) or greater. Having to applylayers of the coating at thicknesses of about 0.5 mils (12.5 microns) orgreater makes it much more difficult to achieve the degree of thicknesscontrol required to achieve the desired corrosion resistant protectionfor the underlying metal substrate.

Other types of corrosion resistant coatings, such as thin diffusioncoatings of alumina or chromia, may be deposited with thicknesses asthin as ˜3 microns. However, these alternative coatings such asdiffusion coatings need to be applied by more complicated processes ortechniques such as physical vapor deposition (PVD), chemical vapordeposition (CVD), pack cementation, etc. These more complicatedprocesses or techniques may cost significantly more to carry out thanthose used to apply aqueous corrosion resistant coating compositions.

The embodiments of the compositions, coatings and methods of thisinvention solve these problems by achieving greater control of coatingthicknesses, e.g., thinner coating thicknesses of about 2.5 mils (63.5microns) or less for corrosion resistant particulates having CTEsapproaching about 4 or about 12, to about 8.5 mils (215.9 microns) orless for corrosion resistant particulates having CTEs approaching thatof the metal substrate, e.g., about 8. This greater coating thicknesscontrol is achieved through the use of corrosion resistant coatingparticulates having a maximum median ESD (M_(p)), in microns, that isrelated to the CTE_(p) of the particulates. For corrosion resistantcomponents useful herein, the maximum median particle size of thecorrosion resistant particulates are defined by one of the followingformulas: (1) for a CTE_(p) of 8 or less, an M_(p) equal to or less than(4.375×CTE_(p))−10; and (2) for a CTE_(p) of greater than 8, an M_(p)equal to or less than (−4.375×CTE_(p))+60, wherein CTE_(p) is theaverage CTE of the particulates and M_(p) is the median ESD of theparticulates. Formulas (1) and (2) typically provide particle sizedistributions of the corrosion resistant particulates such that thedeposited particulates comprise about 20% or less of the maximum coatingthickness. More typically, the maximum median particle size of thecorrosion resistant particulates useful herein are defined by one of thefollowing formulas: (3) for a CTE_(p) of 8 or less, an M_(p) equal to orless than (2.1875×CTE_(p))−5; and (4) for a CTE_(p) of greater than 8,an M_(p) equal to or less than (−2.1875×CTE_(p))+30. Formulas (3) and(4) typically provide particle size distributions of the corrosionresistant particulates such that the deposited particulates compriseabout 10% or less of the maximum coating thickness.

In certain embodiments, it may also be desirable to use corrosionresistant particulates having a maximum particle size. The maximumparticle size (A_(p)), in microns, of the corrosion resistantparticulates typically useful herein is defined by one of the followingformulas: (5) for a CTE_(p) of 8 or less, an A_(p) equal to or less than(10.938×CTE_(p))−25; and (6) for a CTE_(p) of greater than 8, an A_(p)equal to or less than (−10.938×CTE_(p))+150. Formulas (5) and (6)typically provide maximum particle sizes of the corrosion resistantparticulates such that the larger deposited particulates comprise about50% or less of the maximum coating thickness.

In addition to the particle size of the corrosion resistantparticulates, the maximum thicknesses of the corrosion resistant coatingare also controlled by correlating the coating thickness (T_(c)), inmils, with the CTE_(p) of the corrosion resistant particulates used. Forcorrosion resistant coatings useful herein, the maximum coatingthickness is defined by one of the following formulas: (7) for a CTE_(p)of 8 or less, a T_(c) equal to or less than (1.5×CTE_(p))−3.5; and (8)for a CTE_(p) of greater than 8, a T_(c) equal to or less than(−1.5×CTE_(p))+20.5. More typically, the maximum thickness is defined byone of the following formulas: (9) for a CTE_(p) of 8 or less, a T_(c)equal to or less than (0.875×CTE_(p))−2; and (10) for a CTE_(p) ofgreater than 8, a T_(c) equal to or less than (−0.875×CTE_(p))+12.

By controlling, and in the case of corrosion resistant particulateshaving CTEs that vary from that of the metal substrate, reducing theM_(p) (and typically the A_(p)) of the corrosion resistant particulatesin the composition or coating, the maximum thickness of the resultantcoating may be decreased, e.g., to about 2.5 mils (63.5 microns) orless, more typically to about 1.5 mils (38.1 microns) or less, and stillachieve effective corrosion resistant protection. These lower median (aswell as maximum) particle sizes also enable a plurality of thinnerlayers of the composition to be applied with potentially greaterthickness control. This greater thickness control provides the abilityto achieve effective corrosion protection for the metal substrate thatmay be less susceptible to cyclic spallation, especially if there aresignificant differences in CTE between the coating particulates and theunderlying metal substrate.

Embodiments of the compositions may also use corrosion resistantparticulate components having a bimodal particle size distributionscomprising a larger particle size fraction having a median ESD at leastabout 5 times (typically in the range of from about 7 to about 10 times)that of the median ESD of the particulates comprising the smallerparticulate size fraction. The use of corrosion resistant particulatecomponents having a bimodal particle size distribution may increase thepacking efficiency of the corrosion resistant particulate component, andfurther facilitate achieving corrosion resistance with thinner coatingscomprising such corrosion resistant particulate components. Embodimentsof the compositions of this invention may also employ compositionscomprising corrosion resistant particulates of various ceramicsand/metals of differing CTE values for depositing a plurality of coatinglayers, further increasing the ability to make such coatings moreCTE-compatible with the underlying metal substrate.

The various embodiments of turbine components having the corrosionresistant coating of this invention are further illustrated by referenceto the drawings as described hereafter. Referring to FIG. 1, a turbineengine rotor component 30 is provided that can be of any operable type,for example, a turbine disk 32 or a turbine seal element 34. FIG. 1schematically illustrates a stage 1 turbine disk 36, a stage 1 turbineblade 38 mounted to the turbine disk 36, a stage 2 turbine disk 40, astage 2 turbine blade 42 mounted to the turbine disk 40, a forwardturbine seal 44 that also functions as a forward blade retainer forblade 38, an aft turbine seal 46, and an interstage turbine seal 48 thatalso functions as a forward blade retainer for blade 42, an aft bladeretainer 50 for blade 38 that is held in place by seal 48, and an aftblade retainer 52 for blade 42. Any or all of these turbine disks 32(e.g., stage 1 turbine disk 36 and a stage 2 turbine disk 40), turbineseal elements 34 (e.g., forward turbine seal 44, aft turbine seal 46,and interstage turbine seal 48) and/or blade retainers 50/52, or anyselected portion thereof, can be provided with the ceramic corrosionresistant coating of this invention, depending upon whether corrosion isexpected or observed.

Referring to FIG. 2, the metal substrate 60 of the turbine engine rotorcomponent 30 may comprise any of a variety of metals, or more typicallymetal alloys, including those based on nickel, cobalt and/or ironalloys. Substrate 60 typically comprises a superalloy based on nickel,cobalt and/or iron. Such superalloys are disclosed in variousreferences, such as, for example, commonly assigned U.S. Pat. No.4,957,567 (Krueger et al), issued Sep. 18, 1990, and U.S. Pat. No.6,521,175 (Mourer et al), issued Feb. 18, 2003, the relevant portions ofwhich are incorporated by reference. Superalloys are also generallydescribed in Kirk-Othmer's Encyclopedia of Chemical Technology, 3rd Ed.,Vol. 12, pp. 417-479 (1980), and Vol. 15, pp. 787-800 (1981).Illustrative nickel-based superalloys are designated by the trade namesInconel®, Nimonic®, René® (e.g., René® 88 and René® 104 alloys), andUdimet®.

Substrate 60 more typically comprises a nickel-based alloy, andparticularly a nickel-based superalloy, that has more nickel than anyother element. The nickel-based superalloy may be strengthened by theprecipitation of gamma prime or a related phase. A nickel-basedsuperalloy for which the ceramic corrosion resistant coating of thisinvention is particularly useful is available by the trade name René 88,which has a nominal composition, by weight of 13% cobalt, 16% chromium,4% molybdenum, 3.7% titanium, 2.1% aluminum, 4% tungsten, 0.70% niobium,0.015% boron, 0.03% zirconium, and 0.03 percent carbon, with the balancenickel and minor impurities.

Prior to forming the corrosion resistant coating 64 of this invention onthe surface 62 of metal substrate 60, surface 62 is often pretreatedmechanically, chemically or both to make the surface more receptive forcoating 64. Suitable pretreatment methods include grit blasting, with orwithout masking of surfaces that are not to be subjected to gritblasting (see commonly-assigned U.S. Pat. No. 5,723,078 to Nagaraj etal, issued Mar. 3, 1998, especially col. 4, lines 46-66, which isincorporated by reference), micromachining, laser etching (see U.S. Pat.No. 5,723,078 to Nagaraj et al, issued Mar. 3, 1998, especially col. 4,line 67 to col. 5, line 3 and 14-17, which is incorporated byreference), treatment with chemical etchants such as those containinghydrochloric acid, hydrofluoric acid, nitric acid, ammonium bifluoridesand mixtures thereof, (see, for example, U.S. Pat. No. 5,723,078 toNagaraj et al, issued Mar. 3, 1998, especially col. 5, lines 3-10; U.S.Pat. No. 4,563,239 to Adinolfi et al, issued Jan. 7, 1986, especiallycol. 2, line 67 to col. 3, line 7; U.S. Pat. No. 4,353,780 to Fishter etal, issued Oct. 12, 1982, especially col. 1, lines 50-58; and U.S. Pat.No. 4,411,730 to Fishter et al, issued Oct. 25, 1983, especially col. 2,lines 40-51, the relevant portions of which are incorporated byreference), treatment with water under pressure (i.e., water jettreatment), with or without loading with abrasive particles, as well asvarious combinations of these methods. Typically, the surface 62 ofmetal substrate 60 is pretreated by grit blasting where surface 62 issubjected to the abrasive action of silicon carbide particles, steelparticles, alumina particles or other types of abrasive particles. Theseparticles used in grit blasting are typically alumina particles andtypically have a particle size of from about 600 to about 35 mesh (fromabout 25 to about 500 micrometers), more typically from about 360 toabout 35 mesh (from about 35 to about 500 micrometers).

The corrosion resistant coating 64 may be formed on metal substrate 60by any method comprising the steps of: (a) depositing at least one layerof the corrosion resistant coating composition on metal substrate 60;and (b) curing the deposited coating composition at a temperature thatcauses the corrosion resistant particulate component and glass-formingbinder component to form at least one layer of the corrosion resistantcoating 64 that is adjacent to metal substrate 60 and comprises anamorphous, glassy matrix of binder to which the particulates in theparticulate component are embedded in, encapsulated in, enclosed by, orotherwise adhered to. The corrosion resistant coating composition may bedeposited in solid form, e.g., as a flowable solid, as a cast tape(e.g., a cast tape formed as a layer or plurality layers of particulatesadhered together as a coherent mass or matrix by the binder, with orwithout a supporting structure such as a film, strip, etc.), etc, toprovide a solid uncured layer of the composition comprising theparticulates and binder component.

More typically, the coating composition is deposited as a liquid, e.g.,an aqueous coating composition. Liquid corrosion resistant coatingcompositions of this invention may be deposited over or on substrate 60by any manner of application for depositing liquids including pouring,flowing, dipping, spraying, rolling, etc., to provide an uncured layerof the composition comprising the particulates and binder component.This deposited solid or liquid uncured composition layer is then cured,typically by heating to a temperature of at least about 250° F. (121°C.), more typically at least about 500° F. (260° C.) to form corrosionresistant coating 64. Coating 64 may be formed to any thickness up tothe maximum thickness (T_(c)) as previously defined by formulas (6)through (10).

As illustrated in FIG. 3, typically only a portion of the surface ofthese turbine disks/shafts, seals and/or blade retainers are providedwith the corrosion resistant coating 64 of this invention. FIG. 3 showsa turbine disk 32 having an inner generally circular hub portionindicated as 74 and an outer generally circular perimeter or diameterindicated as 78, and a periphery indicated as 82 that is provided with aplurality of circumferentially spaced slots indicated as 86 forreceiving the root portion of turbine blades such as 38, 42. While thecorrosion resistant coating 64 may be applied to the entire surface ofdisk 70, it is typically needed only on the surface of outer diameter78, as well as blade slots 86.

Coating 64 may be formed as a single layer, or may be formed as aplurality of layers. In forming a plurality of layers in coating 64,each respective layer may be formed by depositing a coating compositionand then curing the deposited composition, with the layers being builtup by depositing new portions of a coating composition on the underlyinglayer that was previously formed. A least one of the layers comprisingcoating 64 is formed from embodiments of the corrosion resistant coatingcomposition of this invention and is adjacent to metal substrate 60,with other layers being formed from embodiments of the corrosionresistant coating composition of this invention or from other coatingcompositions. The respective layers of coating 64 may have the same ordiffering thicknesses. For example, when coating 64 comprises aplurality of layers, these layers typically tend to decrease inthickness in the direction from the inner layers (i.e., those closer tosubstrate 60) to the outer layers (i.e., those layers further away fromsubstrate 60). The coating composition used in forming each of therespective layers may have the same or differing levels of particulatecomponent and glass-forming binder component, as well as the same ordiffering types of particulates in the particulate component.

The coating composition used in forming each of the respective layersmay also have the same or a differing binder component, for example,magnesium phosphate in the inner layers and aluminum phosphate in theouter layers. In addition, the level of particulates in the particulatecomponent of the coating composition may differ in the respectivelayers, and typically increases from the inner layers to the outerlayers. For example, the inner layer or layers adjacent to the metalsubstrate may be formed from embodiments of the corrosion resistantcoating compositions of this invention that comprise a higher level oramount of corrosion resistant particulates (e.g., yttria-stabilizedzirconia or hafnia particulates) having a higher CTEs with a better CTEmatch with the metal substrate, while the outer layer or layers notadjacent to the metal substrate may comprise a higher level or amount ofcorrosion resistant particulates (e.g., alumina particulates) havinglower CTEs.

Each layer of coating 64 deposited may be cured to the same or differentdegrees. If desired, an outer glassy sealant layer may be formed forcoating 64 by depositing and curing a composition that is similar to orconsists essentially of a glass-forming binder component that issubstantially free of the particulate component, e.g., a sealantcomposition. Such outer glassy sealant layers may be formed fromcommercially available sealant products, for example, Alseal 598 (fromCoatings for Industry, Inc.), SermaSeal TCS or SermaSeal 570A (fromSermatech International), etc.

An embodiment of a corrosion resistant coating of this inventioncomprising a plurality of layers is shown in FIG. 4 and is indicatedgenerally as 164. As shown in FIG. 4 coating 164 comprises an innerlayer 168 that is adjacent to and overlaying metal substrate 60, and isformed from a corrosion resistant coating composition of this invention.Inner layer 168 is relatively thick and typically comprises from about10 to about 90%, more typically from about 25 to about 75%, of the totalcoating thickness. The particulate component comprising inner layer 168also typically has a greater level or amount of particulates havinghigher CTE values to provide a better CTE match with substrate 60.

Coating 164 also comprises an intermediate layer indicated generally as172 adjacent to and overlaying inner layer 168. Intermediate layer 172may be relatively thinner, especially relative to inner layer 168.Intermediate layer 172 typically comprises from about 10 to about 90%,more typically from about 25 to about 75%, of the total coatingthickness. The particulate component of intermediate layer 172 can alsocomprise an increased amount or level of particulates having lower CTEvalues than that present in inner layer 168 because there is less of aneed for a CTE match with inner layer 168.

As shown in FIG. 4, coating 164 may further comprise an outer layerindicated generally as 176 adjacent to and overlaying intermediate layer172. (In the absence of layer 176, layer 172 would become the outerlayer of coating 164, i.e., overlaying and directly adjacent to innerlayer 168.) This outer layer 176 can comprise a particulate component,but is typically substantially free of particulates. Typically, outerlayer 176 is formed from a sealant composition or a composition thatconsists essentially of, or entirely of, a glass-forming bindercomponent (i.e., is substantially free of particulates) to form a glassyouter sealant layer. Outer layer 176 is also typically the thinnestlayer of coating 164, especially when substantially free ofparticulates. Typically, outer layer 176 has a thickness of from about0.01 to about 1 mils (from about 0.3 to about 25.4 microns), moretypically from about 0.05 to about 0.5 mils (from about 1.3 to about12.7 microns).

While the above embodiments have been described in the context ofcoating turbine engine disks, this invention can be used to formcorrosion resistant coatings, as described above, on the surfaces ofvarious other turbine engine rotor components, including turbine shaftsand seals, exposed to oxygen and other corrosive elements at elevatedtemperatures, turbine components comprising airfoils, for exampleturbine blades and vanes, etc. The corrosion resistant coatings of thisinvention can also be applied during original manufacture of thecomponent (i.e., an OEM component), after the component has been inoperation for a period of time, after other coatings have been removedfrom the component (e.g., a repair situation), while the component isassembled or after the component is disassembled, etc.

While specific embodiments of this invention have been described, itwill be apparent to those skilled in the art that various modificationsthereto can be made without departing from the spirit and scope of thisinvention as defined in the appended claims.

1. A composition comprising: a glass-forming binder component; and aparticulate corrosion resistant component adhered to the glass-formingbinder component and comprising corrosion resistant particulates having:a CTE_(p) of at least about 4 and being solid at a temperature of about1300° F. or greater; and a maximum median particle size defined by oneof the following formulas: (1) for a CTE_(p) of 8 or less, an M_(p)equal to or less than (4.375×CTE_(p))−10; and (2) for a CTE_(p) ofgreater than 8, an M_(p) equal to or less than (−4.375×CTE_(p))+60,wherein CTE_(p) is the average CTE of the corrosion resistantparticulates and wherein M_(p) is the median equivalent sphericaldiameter (ESD), in microns, of the corrosion resistant particulates;wherein the corrosion resistant coating has a maximum thickness definedby one of the following formulas: for a CTE_(p) of 8 or less, an T_(c)equal to or less than (1.5×CTE_(p))−3.5; and for a CTE_(p) of greaterthan 8, an T_(c) equal to or less than (−1.5×CTE_(p))+20.5; whereinT_(c) is the thickness, in mils, of the corrosion resistant coating. 2.The composition of claim 1, wherein the corrosion resistant particulateshave a CTE_(p) in the range of from about 4 to about
 12. 3. Thecomposition of claim 2, wherein the maximum median particle size isdefined by one of the following formulas: (3) for a CTE_(p) of 8 orless, an M_(p) equal to or less than (2.1875×CTE_(p))−5; and (4) for aCTE_(p) of greater than 8, an M_(p) equal to or less than(−2.1875×CTE_(p))+30.
 4. The composition of claim 2, wherein thecorrosion resistant particulates have a maximum particle size defined byone of the following formulas: (5) for a CTE_(p) of 8 or less, an A_(p)equal to or less than (10.938×CTE_(p))−25; and (6) for a CTE_(p) ofgreater than 8, an A_(p) equal to or less than (−10.938×CTE_(p))+150,wherein A_(p) is the maximum particle size, in microns.
 5. Thecomposition of claim 1, wherein the corrosion resistant particulatescomprise a ceramic.
 6. The composition of claim 5, wherein the ceramicis a metal oxide, carbide, nitride, or combination thereof.
 7. Thecomposition of claim 6, wherein the corrosion resistant particulatescomprise alumina, chromia, magnesia, hafnia, or a yttria-stabilizedzirconia or hafnia.
 8. The composition of claim 1, wherein the corrosionresistant particulates comprise a metal.
 9. The composition of claim 8,wherein the metal is an overlay metal alloy having the formula MCr, MAl,MCrAl, MCrAlX, or MAlX, wherein M is iron, cobalt, nickel, or an alloythereof and wherein X is hafnium, zirconium, yttrium, tantalum,platinum, palladium, rhenium, silicon, lanthanum, or a combinationthereof.
 10. The composition of claim 9, wherein the overlay metal alloycomprises a MCrAlY alloy, wherein M is nickel, cobalt or a nickel-cobaltalloy.
 11. The composition of claim 1, wherein the corrosion resistantparticulates comprise a combination of a ceramic and a metal.
 12. Thecomposition of claim 1, wherein the glass-forming binder comprises aphosphate-containing binder component.
 13. The composition of claim 12,wherein the phosphate-containing binder component comprises one or moreof an aluminum phosphate, a magnesium phosphate, or a chromiumphosphate.
 14. The composition of claim 12, wherein thephosphate-containing binder component is substantially free of otherbinder materials.
 15. The composition of claim 1, which furthercomprises a liquid carrier component.
 16. The composition of claim 15,wherein the liquid carrier component comprises water.
 17. A compositioncomprising: a glass-forming binder component; and a particulatecorrosion resistant component adhered to the glass-forming bindercomponent and comprising corrosion resistant particulates having: aCTE_(p) of at least about 4 and being solid at a temperature of about1300° F. or greater; and a maximum median particle size defined by oneof the following formulas: (1) for a CTE_(p) of 8 or less, an M_(p)equal to or less than (4.375×CTE_(p))−10; and (2) for a CTE_(p) ofgreater than 8, an M_(p) equal to or less than (−4.375×CTE_(p))+60,wherein CTE_(p) is the average CTE of the corrosion resistantparticulates and wherein M_(p) is the median equivalent sphericaldiameter (ESD), in microns, of the corrosion resistant particulates;wherein the corrosion resistant coating has a maximum thickness definedby one of the following formulas: for a CTE_(p) of 8 or less, an T_(c)equal to or less than (1.5×CTE_(p))−3.5; and for a CTE_(p) of greaterthan 8, an T_(c) equal to or less than (−1.5×CTE_(p))+20.5; whereinT_(c) is the thickness, in mils, of the corrosion resistant coating andwherein the particulate corrosion resistant component comprises bimodalparticle size distribution having from about 60 to about 95% by volumeof a larger particle size fraction and from about 5 to about 40% byvolume of a smaller particulate size fraction, wherein the largerparticle size fraction comprises particulates having a median ESD atleast about 5 times that of the median ESD of the particulatescomprising the smaller particulate size fraction.
 18. The composition ofclaim 17, wherein the larger particle size fraction comprisesparticulates having a median ESD in the range of from about 7 to about10 times that of the median ESD of the particulates comprising thesmaller particulate size fraction.